This invention is directed to a method for ultrasonically bonding or joining thermoplastic materials. More specifically, this invention is directed to preheating substantially melt-compatible thermoplastic materials prior to ultrasonic bonding to provide an ultrasonic bond having an increased bond strength.
Disposable garments, such as adult incontinence wear as well as infant and children""s diapers, swim wear and training pants, typically include materials which are joined together and connected using an ultrasonic process. For example, a training pant may have a front side panel and a back side panel which are joined together at a side seam to provide a complete side panel. The side panels are typically connected at the side seam using an ultrasonic process. However, the ability to form strong ultrasonic bonds using conventional processes is limited by several factors, including the process converting speeds or production line speeds, bonding time or dwell time, and the thickness and/or basis weight of the materials being bonded. For example, for particular materials, such as spunbond laminate materials, as the production line speed increases, the dwell time decreases and a strong ultrasonic bond may not form properly. Insufficient bond strength can severely limit potential product converting speeds. Further, for these particular materials, the hot melt adhesives used to bond the spunbond materials and the elastic filaments or fibers may inhibit the effectiveness of conventional ultrasonic bonding processes.
Several early attempts to improve the ultrasonic bonding process focused on the mechanics of the ultrasonic horn, such as, for example, increasing the energy available from the ultrasonic horn by increasing the horn vibrational amplitude or other design features. However, the mechanical design of the ultrasonic horn may limit the maximum energy that can be delivered by the ultrasonic system to the materials being bonded, thus making it difficult to further enhance the bonding capability of the ultrasonic bonder by modifying the ultrasonic horn and/or the amplifier design.
Thus, there is a need or desire for an ultrasonic bonding process that provides sufficiently strong bonds at increased production line speeds and corresponding decreased dwell times.
There is a need or desire for a controllable ultrasonic bonding process that provides ultrasonic bonds of varying predetermined strength.
There is a need or desire for an ultrasonic bonding process that provides sufficiently strong intermittent bonds, such as bonds to provide side seams which connect the front side panel and back side panel of personal care garments.
In response to the difficulties and problems discussed above, a method for ultrasonically bonding thermoplastic materials, such as, for example nonwoven webs and spunbond laminates, wherein the ultrasonic bonding method results in a substantial increase in the bond strength established between the materials, has been discovered. Preheating at least one of the materials prior to the ultrasonic bonding process results in an increase in the bond strength established between the materials.
Suitable materials include, but are not limited to, elastic spunbond laminates, for example vertical filament stretch-bonded laminates (VF SBL) and continuous filament stretch-bonded laminates (CF SBL). Additionally, films, woven and other nonwoven webs and/or solid blocks of ultrasonically bondable substrates may also be suitable materials. Additionally, many product applications are contemplated for this invention, wherein sufficient peel strength, shear strength, creep resistance, and/or multilayer construction is required.
In particular embodiments, preheating the materials prior to entering a main ultrasonic bonding apparatus results in a substantial increase in ultrasonic bond strength. The bond strength increases proportionately as the substrate feed temperature increases. There is a consistent trend between substrate temperature and ultrasonic bond strength over a range of about 70xc2x0 F. to about 250xc2x0 F. At higher initial or preheating temperatures, the strength of the subsequent ultrasonic bond may approach an ultimate tensile strength of the material. Alternatively, the materials may be cooled prior to entering the main ultrasonic bonding apparatus to a temperature of about 40xc2x0 F. to about 70xc2x0 F., thereby decreasing the strength of the subsequent ultrasonic bond.
Desirably, at least one substrate is preheated prior to entering the main ultrasonic bonding apparatus using a separate ultrasonic preheating unit. In one embodiment of this invention, the ultrasonic preheating unit includes an ultrasonic horn and an anvil having a smooth peripheral surface. As the materials are fed through a nip formed between the ultrasonic horn and the smooth anvil, the materials are preheated to an initial or preheating temperature and compressed, without substantially bonding or only partially bonding the materials. Other means known to those having ordinary skill in the art may be used for preheating the materials prior to entering the main ultrasonic bonding apparatus, such as hot air, heated blocks or heated rollers, infrared, laser, RF and microwave preheating means and equivalents thereof.
Decoupling the preheating process from the ultrasonic bonding process enables the bond strength to be greatly increased over the capability of the main ultrasonic bonding apparatus alone. It also allows the bond strength to be controllably xe2x80x9cdialed inxe2x80x9d between essentially no bond to a bonding level which may approach the ultimate tensile strength of the materials, irrespective of the bonding speed and bonding time.
With the foregoing in mind, it is a feature and advantage of the present invention to provide a method for ultrasonically bonding thermoplastic materials, for example melt-compatible thermoplastic materials, wherein at least one of the materials is preheated before the materials are ultrasonically bonded to substantially increase the ultimate bond strength established between the materials.
It is further a feature and advantage of the invention to provide a method for ultrasonically bonding thermoplastic materials, wherein the bond strength can be controlled irrespective of the bonding or production line speed and/or the bonding or dwell time.
It is a feature and advantage of the invention to provide an ultrasonic bonding method that provides sufficiently strong bonds.